PHAs and other thermoplastic polyesters represent potential raw materials for a myriad of useful products which include melt-spun fibers from which non-woven products can be produced for medical gowns, masks etc., blown and cast films for compostable grocery and garbage bags, injection-molded bottles for health and personal care products, extrusion coatings on paper/paperboard for biodegradable/compostable fast-food containers and lawn and grocery sacks/bags etc. In the processes to produce such products, it is crucial to achieve line speeds, cycle times, and other processing parameters that are economically desirable. These parameters can be influenced to some extent by the use of nucleants, also referred to as nucleating agents, which promote crystallization of the molten or glassy resin resulting in a more rapid loss of tackiness and concurrent increase in mechanical strength. Thus, there is economic incentive to discover and develop efficient nucleating agents for PHAs and other thermoplastic polyesters.
PHAs are biopolymers synthesized inside the cell of many microorganisms as storage substances for energy and carbon and have the great advantage that they are compostable and biologically degradable (For review, see Anderson and Dawes, Micro. Rev. 54 (4): 450-472, 1990). PHAs are thermoplastic polyesters which can be processed into films, coatings, and shaped articles among other applications. Nevertheless, their processing frequently presents difficulties because of low rates of crystallization and poor clarity, limiting their use in many applications. In addition, PHA end-products are typically characterized by poor mechanical properties including a tendency to embrittlement with age after melt processing.
PHAs are being developed as a sustainable solution to address the issue of solid waste generated by the use of materials produced from fossil fuel. Some of the markets and applications most amenable to the use of PHAs include one-time-use disposables, packaging and biomedical applications.
The rate of crystallization of many PHAs is slow due to low nucleation density leading to long cycle times for thermal processing of PHAs into articles of manufacture. Production of shaped articles, films, coatings and other PHA-derived products therefore often requires the use of PHAs containing nucleating agents to increase the nucleation density and rate of crystallization, thereby allowing the polymers to be processed more economically. In addition, such agents can sometimes improve the physical and mechanical properties of the processed articles. Conventional nucleating agents include, for example, talc, micronized mica, calcium carbonate, boron nitride (EP-A-0291024), ammonium chloride (WO-A-9119759), and others.
U.S. Pat. No. 5,296,521 describes polyester compositions having increased crystallization rates comprising thermoplastic polyester resins (e.g. PET) and 0.5 to about 5 wt % of nucleating agent of the formula RO[P(O)(Ph)(CH2)mO]nH where R is an alkali or alkaline earth metal; m is 1, 2, or 3; n takes an average value within the range of 1 to 5. The nucleating agent can be optionally mixed with the acid or ester form (R=H or alkyl) providing at least 50 mole % of the nucleating agent is in the salt form. The nucleating agent is preferably in the form of the sodium salt (e.g. sodium salt of hydroxymethylphenyl phosphinic acid or sodium salt of oligomethylene phenyl phosphinic acid).
U.S. Pat. No. 4,536,531 describes use of carboxylic salts of metals of Group I and II in the Periodic Table as nucleating agents for polyesters exemplified by metal salts of aliphatic monocarboxylic acids such as acetic acid, propionic acid, caproic acid, palmitic acid, stearic acid, oleic acid, behenic acid, montanic acid etc. Suitable metals are sodium, potassium, lithium, magnesium, calcium, barium, and zinc. In these carboxylic acid salts, it is unnecessary that all the carboxyl groups be converted into salt form, but a part of the carboxyl groups may be in a salt form and the remaining groups may be in a free acid or ester form.
JP 48074550 describes use of metal salts of aryl phosphonates as nucleating agents for polyesters (e.g. PBT). Use of 0.5 wt % zinc phenylphosphonate in PBT gave a faster molding time and improved properties compared with PBT alone. Calcium benzylphosphonate and aluminum phenylphosphonate gave similar results.
U.S. Pat. No. 5,061,743 discloses a preferred PHA nucleant made by dry blending cyclohexylphosphonic acid (DZB) and zinc stearate with polyhydroxy-butyrate-co-valerate (PHBV). The nucleant is disclosed as particularly advantageous for the nucleation of PHBV having high HV content.
RU 2048484 describes the use of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) in a solution of technical grade methyl phosphate to reduce the concentration of carboxyl groups and increase the thermal stability of polyethylene terepthalate (PET).
Although many of these compounds have shown effectiveness in increasing the nucleation density of PHA, and therefore crystallization rates, certain disadvantages have been associated with their use. Dispersion of particulate nucleants, for example, has been problematic because agglomeration frequently occurs during processing which can generate regions of stress concentration and inhomogeneity in molding. In addition, nucleants such as boron nitride have been found to act as pigments in some situations, particularly in films and injection moldings, giving rise to opaque products where transparent products are generally desired. Furthermore, some nucleant systems include constituents which may be undesirable from an environmental and toxicological standpoint. Thus, there is a need for benign and cost-effective nucleant systems which allow for the production of PHA resins having high crystallinity and excellent moldability, mechanical strength, and dimensional stability.